The present invention relates to fiber optic communication systems, and more particularly to apparatus for transmitting amplitude modulated vestigial-sideband ("AM-VSB") television signals over an optical fiber television system.
Cable television systems currently distribute television program signals via coaxial cable, typically arranged in tree and branch networks. Coaxial cable distribution systems require a large number of high bandwidth electrical amplifiers. For example, 40 or so amplifiers may be required between the cable system headend and an individual subscriber's home.
The use of a television signal comprising amplitude modulated vestigial- sideband video subcarriers is preferred in the distribution of cable television signals due to the compatibility of that format with NTSC television standards and the ability to provide an increased number of channels within a given bandwidth. An undesirable characteristic of AM-VSB transmission, however, is that it requires a much higher carrier-to-noise ratio (CNR) than other techniques, such as frequency modulation or digital transmission of video signals. Generally, a CNR of at least 40 dB is necessary to provide clear reception of AM-VSB television signals.
The replacement of coaxial cable with optical fiber transmission lines in television distribution systems has become a high priority. Production (i.e., commercially available) single mode fiber can support virtually unlimited bandwidth and has low attenuation. Accordingly, a fiber optic distribution system or a fiber-coax cable hybrid would provide substantially increased performance at a competitive cost as compared to prior art coaxial cable systems.
One problem in implementing an optical fiber distribution system, particularly for AM-VSB signals, is that the lasers used to transmit the television signal information inherently introduce distortion, most notably as a result of second and third order harmonics produced by the nonlinearities of the laser.
Amplification of optical signals within a fiber optic network has also been a problem in the attempt to distribute AM-VSB television signals. As noted above, amplifiers are required between a cable system headend and a subscriber's home in order to provide signals to the subscriber at an acceptable power level. Semiconductor optical amplifiers of the type typically used in fiber optic systems produce high levels of distortion products that are not compatible with multi-channel AM-VSB video signals. This is due to the short lifetime of the carrier excited state within the semiconductor optical amplifier. The recombination time of such an amplifier operating near 1.3 .mu.m or 1.5 .mu.m is about 1.2 nanoseconds, which is short compared to the period of a typical AM-VSB subcarrier operating in the cable television band of about 55.25 MHz-1 GHz.
The dependence of second order distortion on carrier lifetime in a semiconductor optical amplifier is discussed in A.A.M. Saleh, et al., "Nonlinear Distortion Due to Optical Amplifiers in Subcarrier-Multiplexed Lightwave Communications Systems", Electronics Letters, Vol. 25, No. 1, pp. 79-80, 1989. As noted in that article, second order nonlinear distortion is a significant problem in proposed lightwave cable television home distribution systems, where the use of semiconductor amplifiers to overcome inevitable distribution losses can potentially degrade system performance appreciably.
The difficulties presented in transmitting multi-channel AM-VSB television signals over fiber optic distribution systems have led others to propose the use of frequency modulation ("FM") instead of the more desirable AM-VSB format. See, e.g., R. Olshansky, et al., "Microwave-Multiplexed Wideband Lightwave Systems Using Optical Amplifiers for Subscriber Distribution", Electronics Letters, Vol. 24, No. 15, pp. 922-923, 1988; R. Olshansky, et al., "Subcarrier Multiplexed Passive Optical Network for Low-Cost Video Distribution", presented at OFC 1989; and W. I. Way, et al., "Carrier-to-Noise Ratio Performance of a Ninety-Channel FM Video Optical System Employing Subcarrier Multiplexing and Two Cascaded Traveling-Wave Laser Amplifiers", presented at OFC 1989. Another proposal has been to convert AM-VSB signals to a digital format for transmission. Digital transmission of AM-VSB television signals over an optical communication link is described in U.S. Pat. No. 4,183,054 to Patisaul, et al., entitled "Digital, Frequency-Translated, Plural-Channel, Vestigial Sideband Television Communication System".
It would be advantageous to provide an apparatus and method for transmitting AM-VSB television signals over a fiber optic distribution system in analog form. It would be further advantageous to provide a means for reducing second order distortion in an amplitude modulated subcarrier optical communication system. The present invention provides such advantages.